1. Field of the Disclosure
The present disclosure relates to an ohmic contact layer and a light emitting device having the same, and more particularly, to an ohmic contact layer which can enhance the light extraction efficiency of a top emission type light emitting device and a light emitting device having the same.
2. Description of the Related Art
The effective supply of current to a semiconductor light emitting device is significantly influenced by an ohmic contact between a semiconductor layer and an electrode. A light emitting device made of a nitride semiconductor, for example, a gallium nitride semiconductor, needs a particularly good ohmic contact. Ohmic contacts of nitride semiconductor light emitting devices are still unsatisfactory in spite of continuous research thereon because of low carrier concentration, high sheet resistance, and low electric conductivity of the p-type gallium nitride. In addition, since internally generated light is emitted through an ohmic electrode layer in a top light emission type light emitting device, the ohmic contact layer must have good electrical characteristics and high light transmittance.
A conventional ohmic contact layer of a top light emission type light emitting device includes a nickel (Ni) layer and a gold (Au) layer sequentially stacked on a second cladding layer. The ohmic contact layer formed of the stacked metal layers has a low specific contact resistance of about 10−3 to 10−4 Ωcm2 after being heat-treated under an oxygen or air atmosphere. However, such a conventional Ni/Au ohmic contact layer has low light extraction efficiency due to its low light transmittance, i.e., about 75% at λ=450 nm. Accordingly, although the ohmic contact layer has low specific contact resistance, the ohmic contact layer cannot be employed in a next generation top light emission type light emitting device having high-output power and high luminance because of its low light transmittance.
To overcome the output power limitations of top emission type light emitting devices, the use of a transparent conductive oxide, for example, ITO, having excellent light transmittance has been suggested [T. Margalith et al., Appl. Phys. Lett. Vol. 74, p 3930 (1999).]
Y. C. Lin et al. discloses a Ni/ITO ohmic contact layer having an 86.6% greater transmittance than a conventional Ni/Au ohmic contact layer used to form a top emission type device having 1.3 times the light output power [Y. C. Lin et al., Solid-State Electronics Vol. 47, p 849 (2003)].
Recently, an electrode surface texturing method has been suggested to maximize the light extraction efficiency of devices. In the electrode surface texturing method, a NiO/ITO ohmic contact layer, for example, is formed on GaN, and then, the ITO electrode is patterned using a plasma etching photolithography method to form holes having diameters of several micrometers in the ITO electrode, thereby increasing the light output power by about 16% [by S.-M. Pan et al., in IEEE Photon. Technol. Left. Vol. 15 p 649 (2003)]. However, since the holes formed in the surface of the electrodes using the electrode surface texturing method have diameters of several micrometers, the light extraction efficiency thereof cannot be maximized. In particular, the device performance may be degraded due to plasma damage to the device during etching.